This invention relates to apparatus for processing digitized video signals used in apparatus such as color television, VCR, video disk player (VDP), video printer or image transmission apparatus and more particularly to a video signal processing circuit suitable for enlarging and displaying video signals.
Digital signal processing techniques have been introduced into electric home appliances, making it possible to perform various special display functions even in apparatus such as TV, VTR and VDP. For example, picture still, segmental picture display and the like have already been commercialized. Recently, as shown in JP-A-61-149987, a function to effect enlargement on the TV screen has been developed. In the aforementioned known example, pictures corresponding to various inputs can be n times enlarged in the vertical direction.
The enlarging method of the aforementioned known example will now be described with reference to FIG. 2. In FIG. 2, 201 designates a horizontal synchronization signal generation circuit, 202 an enlargement coefficient setting circuit for setting a magnification of enlargement, 203 a counter for counting the horizontal synchronization signal in accordance with the magnification of enlargement, 204 a selector for switching between the horizontal synchronization pulse for normal reproduction and that for enlargement, 205 a field discriminator, 206 a clock delay circuit for generating horizontal synchronization pulses under interlaced scanning in accordance with the magnification of enlargement and a field to be enlarged, 207 a clock control circuit for dividing the horizontal synchronization signal into two in the mode of normal reproduction under interlaced scanning, 208 a selector for between selecting the horizontal synchronization pulse in accordance with a structure of the input signal and a control signal for enlarged display of normal display, 209 a vertical address counter, 210 a horizontal address counter, 211 an adder for synthesizing vertical and horizontal addresses, 212 an image memory for storing an image to be enlarged, and 213 an image output terminal. In the known example, the counter 203, clock delay circuit 206 and clock control circuit 207 prepare, in accordance with the magnification of enlargement and the structure of the input signal, horizontal synchronization pulses to be inputted to the luminance (Y) address counter 209 in order that vertical addresses on the image memory 212 be so changed as to produce an enlarged image. For example, under the interlaced scanning of the input signal, vertical addresses are as shown in FIG. 3A in the case of normal display but in the case of 4-times enlargement, vertical addresses are as shown in FIG. 3B. In the case of the normal display under interlaced scanning, the vertical address increases through 00, 02, 04 . . . in the first field and through 01, 03, 05 . . . in the second field. If in this case images of the first field are stored at even addresses in the image memory 212 and images of the second field at odd addresses, accurate reproduction can be effected. On the other hand, in the case of the enlarged display, the vertical address increases through 00, 00, 01, 01, 02, 02 . . . in the first field and advances through 00, 00, 01, 01, 02, 02 . . . also in the second field. Accordingly, an identical image can be reproduced over four lines in the vertical direction to permit a 4 times enlarged display.
In the aforementioned prior art, the image is enlarged by changing the address on the image memory in accordance with the configuration of the input signal and the magnification of enlargement and a correctly enlarged display can be effected when a still picture is stored in the image memory. However, when the enlargement of display according to FIG. 3B is carried out in connection with display of a moving picture as in the case of TV, signals of even addresses indicative of images inherently belonging to the first field and signals of odd addresses indicative of images inherently belonging to the second field are subjected to display within the same first field, with the result that a comb-tooth like image is displayed, producing an unnatural picture.
The state of this disturbance will be described in greater detail with reference to FIGS. 4A to 4D. FIGS. 4A to 4D show the structure of scanning line in the mode of enlargement. FIG. 4A shows an incoming video signal. Scanning lines as viewed exactly laterally thereof are illustrated by circles, and individual scanning lines are named after alphabet and numerical numbers indicated in the circles on the assumption that a field indicated by numerical numbers is the present field, a field indicated by alphabet one field precedes the present field and a field indicated by numerical numbers hatted with dash two fields precedes the present field. FIGS. 4B and 4C show scanning line structures obtained when an image based on scanning lines shown in FIG. 4A is twice enlarged and displayed. FIG. 4B shows an instance where the image is enlarged and displayed according to the method of the aforementioned known example. In the instance shown in FIG. 4B, scanning lines of different fields are subjected to display within the same field and as a result, the comb-tooth like disturbance occurs in a moving picture. To solve this problem, signals of the present field are prepared using incoming signals of the present field and signals of the one-preceding field are prepared using signals of the one-preceding field, providing the structure shown in FIG. 4C. In this case, however, inversion of scanning line position occurs between scanning lines indicated by arrows and turns into disturbance.
In enlargement of a moving picture, even for a display of the same field, the corresponding image stored in the memory sometimes bridges two fields depending on the position on the screen at which the enlargement occurs and picture pattern displacement takes place between upper and lower portions of the image. In the aforementioned prior art example, enlargement at a lower portion of the screen can be effected by changing the initial value of the Y address counter 209. FIG. 4D shows an instance where a portion following the third line is enlarged. In FIG. 4D, lines (above the chained line) of an upper portion are constructed of signals of the two-preceding field and lines of the lower portion are constructed of signals of the present field. Thus, signals of different fields are displayed within the same field, resulting in an unnatural picture.
As described above, the prior art fails to consider enlargement of a moving picture under interlaced scanning such as ordinary television signals and faces problems:
(1) Comb-tooth like disturbance takes place in the image.
(2) Displacement of image takes place between upper and lower portions of the screen.